A large number of the most various devices for the detection of projecting material defects in moving material webs are known from prior art. In many cases, profile sensor devices are used for this purpose which are arranged above or below the material web to be monitored so as to monitor said material web from above or below. Profile sensor devices of this type need to be arranged in a horizontal cascade which makes them extremely complex and expensive. The same applies to visual monitoring systems.
Furthermore, simple light barriers are used in particular in the processing of narrow webs. These systems are however unable to detect the position of the defect. Moreover, the system often needs to be mechanically adjusted to different material web thicknesses.
Another problem of these known solutions is that false alarms occur frequently. Material defects of this type may result in severe damages during subsequent processing of the material web. The damages may occur on processing devices, on the material web itself or on the surroundings. In many cases, the material defects cause damages in particular in digital printing devices such as inkjet digital printing devices since these devices usually comprise sensitive fine ceramic components.